Continents and Oceans

If you tabulate the elevations of all parts of the globe--including the ones covered by water--an interesting fact emerges. Those elevations--it turns out--are not smoothly distributed, but tend to cluster in one of two neighborhoods.

Most dry land has a modest elevation above sea level, while most of the ocean floor is about 3 kilometers (or 2 miles) lower down. The area of in-between depths, e.g. where the ocean is about 1 kilometer deep, is much less. An atlas will show that in the oceans around the continental US, for instance, for a certain distance from land the depth slowly increases, but then the sea-bottom plunges down steeply to the lower level, where it stays.

What does this mean? It means that the surface of the Earth is not a single terrain, varying smoothly, part of which happens to stick out above water. Rather, its regions belong to one of two types. The oceans tend to be uniformly deep, while the continents are separate chunks, thick enough to rise above water (or, at their edges, be covered by shallow seas).

Alfred Wegener

Continental Drift

Alfred Wegener, a German arctic explorer and geophysicist who lived in the early 1900s, was struck by the resemblance between the continents and ice-floes in the arctic oceans, resulting from the break-up of sheets of floating sea-ice. Just as ice-floes which have broken apart match along the line of break, so did the edges of some continents match, e.g. Africa and South America. Maybe those land masses, too, used to be together?

Wegener found other corresponding matches, e.g. between rock formations along matching edges, and in 1918 he proposed his theory of "continental drift"--that continents, like ice floes, drifted from one location to another. He believed the continents floated on deeper layers below them, which over millions of years gave way like a thick fluid and made the drift possible. The energy source was supposedly the internal heat of the Earth.

Wegener's idea encountered enormous resistance from established geophysicists. Sir Harold Jeffreys in Britain, in particular, pointed out that the deeper layers were not nearly fluid enough and would strongly resist the proposed motion. After Wegener died on an arctic expedition in 1930, only a handful of loyal supporters continued to promote his ideas. More evidence was needed, and it came from the Earth's magnetism.

Ocean Floor Magnetism

Mid-Atlantic Ridge

In the 1950s electronic magnetometers were developed. Unlike the older instruments, based on the compass needle, these could be towed behind an airplane or a ship. Oil companies were soon using them aboard airplanes, mapping the weak magnetism of rocks to help locate oil deposits. On land, the patterns of this magnetism seemed jumbled, with no meaningful order.

Extending those measurements to the oceans, around 1960, revealed a surprising difference. In the ocean floor the magnetization was orderly, arranged in long strips. The strips on the Atlantic ocean floor, in particular, all seemed parallel to the "mid-Atlantic ridge." That is a volcanic ridge running roughly north-to-south (with some zigs and zags), halfway between Europe-Africa and America. It is marked by the focus-points of earthquakes and by some volcanic islands, and more recently it was explored by research submarines, which have at times observed lava oozing out at its crest.

Ocean floor magnetization (USGS figure)

Not only were the magnetic strips lined-up with the central ridge, but their structure and distribution seemed remarkably symmetric on both sides: if (say) a narrow-wide pair of strips was observed at a certain distance east of the ridge, its mirror image was also found at about the same distance to the west.

If the sea-floor was moving, then continents adjoining them might share that motion, just as Wegener had guessed. The main difference now seems to be that rather than pushing their way through a semi-fluid on which they float, the continents (or some of them) ride on top of "conveyer belts" in that fluid. These are the "plates" which emerge at mid-ocean and go down again (at least in some cases) at the deep oceanic trenches, like the ones found near Japan or in the Caribbean Sea.

The science of the shaping of the Earth's crust goes by the name "tectonics," and the process described here is the essence of "plate tectonics" by the Earth's crust consists of distinct plates which are continually rearranged, sometimes carrying along continents or parts of continents. The entire motion is indeed driven by the Earth's internal heat.

The Pacific plate bordering California, for instance, is slowly rotating, moving northwards. The edge of California is attached to that plate and also moves northwards, but the bulk of the continent does not. The juncture between the two, where one slips by the other, follows in part the famous San Andreas fault.

Further Reading

In 1996 the US Geological Survey (USGS) published a book "This Dynamic Earth" by W. Jacquelyne Kious and Robert I Tilling. This book, in its entirety, is on the web, and can be accessed here. In clear language with many illustrations (including the ones shown above), it tells the story of plate tectonics much more completely than could be done here. One of its many interesting sections describes the life and work of Alfred L. Wegener.